The present invention relates generally to the field of sphygmometry, and moe particularly to sphygmometric blood pressure monitoring.
The prior art is replete with devices for measuring the blood pressure of a living subject. An old and simple device is a pressurizable cuff used in combination with a mercury manometer which reads pressure in the cuff and a stethoscope which is used to listen to Korotkoff sounds. More complicated methods and apparatus based on the same principle of listening to the Korotkoff sounds replace the mercury manometer with a mechanical or electromechanical pressure gauge and utilize microphonic detection of the Korotkoff sounds which are analyzed electrically. In another advanced method for measuring blood pressure, the distance from a blood pressure cuff to the wall of an artery is accurately determined by measuring Doppler shifts of sound waves reflected by the artery. The distance to the artery varies as a function of pressure within the somewhat pliable walls thereof. In yet other methods for measuring blood pressure intrusive devices may be inserted directly into the blood vessels.
Oscillometric methods of determining blood pressure are also known in the art. Several recent contributions to the art of oscillometric blood pressure monitoring have been provided by one or more of the present inventors as represented by U.S. Pat. No. 3,903,872 issued to William T. Link on Sept. 9, 1975 for Apparatus and Process for Producing Sphygmometric Information; U.S. application Ser. No. 578,047 filed May 15, 1975 by Link et al for Apparatus and Process for Determining Systolic Pressure and now U.S. Pat. No. 4,009,709; and more recently U.S. application Ser. No. 754,133, filed Dec. 27, 1976, by William T. Link for Apparatus and Process Using Second Derivative of Oscillometric Waveform for Producing Sphygmometric Information, each of the aforementioned patents and/or applications being incorporated herein by reference.
The U.S. Pat. No. 3,903,872 discloses a technique and apparatus for the accurate determination of diastolic blood pressure by employing an oscillometric technique in which a pulsating signal derived from a pressurizable cuff is differentiated to provide a first time derivative (P.sub.ac), which first time derivative is then maximized as a function of the applied pressure to obtain diastolic pressure.
The U.S. application Ser. No. 578,047 now U.S. Pat. No. 4,009,709 discloses a relationship between the length of the pressure cuff and that portion of an embraced blood vessel which undergoes volumetric change at systolic pressure, such as to permit a fairly accurate determination of systolic pressure to be made as a function of the amplitude of the pulsatile pressure sensed in the cuff. In the normally encountered situation the systolic pressure is recognized as that at which the amplitude of the pulsating component (P.sub.ac) of pressure sensed in the pressurized cuff is about 1/2 the maximum pulse amplitude for an applied pressure greater than the pressure at which the maximum pulsatile pressure occurs.
In addition to utilizing the maximized negative first time derivative of the pulsatile pressure to establish the diastolic pressure of a blood vessel, it is further appreciated that the applied pressure region above the diastolic pressure necessarily embraces the systolic pressure and may be considered to be a nonspecific indicator of the range in which the systolic pressure may be found. Further, analysis of the amplitude of the pulsatile pressure as the applied pressure is varied at least across the diastolic-systolic range, in addition to revealing the systolic pressure, may also be utilized to generally indicate a range for the diastolic pressure which corresponds with the "left-hand shoulder" of the envelope which embraces the plot of the pulse pressure waveform across the applied pressure range.
In the aforementioned application Ser. No. 754,133 by Link, there is disclosed an alternative technique for determining both diastolic and systolic pressure. This technique utilizes the second time derivative (P.sub.ac) of the sensed pulsatile pressure by noting when negative spikes of the second time derivative first and last exceed some minimal negative threshold level as applied cuff pressure is differed across the diastolicsystolic range. The applied pressures corresponding with the onset and termination of such threshold-exceeding negative second time derivative signals correspond substantially with diastolic and systolic pressures of the blood vessel.
However, in essentially all of the preceding techniques (including oscillometry) for measuring blood pressure, including invasive techniques, there exists the problem that signal artifacts arising from random muscular activity and the like may result in inaccurate or invalid pressure readings. A variety of techniques have been employed for rejecting artifacts from the sensed signal in an effort to improve the accuracy of the various equipment. For instance, such techniques have rejected pulses of too great or too small amplitude, and/or pulses which occur too early or too late relative to some time reference. Further, several pulses may be averaged so that the effect of any artifact is decreased. Further still, the blood pressure waveforms used in oscillometric techniques may be correlated with waveforms obtained from an electrocardiographic analysis in an effort to reject artifact signals. Although the foregoing artifact rejection and/or moderation techniques contribute to increased accuracy in blood pressure monitoring equipment, they may be expensive to employ and/or certain artifacts may in any event continue to appear.
Accordingly, it is a principle object of the invention to provide improved apparatus and process for identifying and/or rejecting artifacts in blood pressure measurements.